This invention relates to a photopolymerisation and photocrosslinking process, and in particular to such a process for the production of images.
Conventionally, production of an image by means of photopolymerisation is achieved by coating a support with a solution in a volatile organic solvent of a photopolymerisable substance, causing or allowing the solvent to evaporate so leaving a film of the photopolymerisable substance, irradiating the film with actinic radiation as through an image whereby the parts of the film struck by the irradiation become photopolymerised (and less soluble) while those parts shielded from the irradiation remain substantially unaffected, then dissolving away the unirradiated, unphotopolymerised parts of the film by means of a suitable solvent which does not dissolve the irradiated, photopolymerised parts. This last stage is conventionally known as "development".
It would be desirable to have a process in which a layer of a photopolymerisable substance were applied to a support and this layer were converted into a substantially solid, nontacky state, for irradiation, without the use of organic solvents. Not only would, in this stage, the use be avoided of solvents which might present problems of toxicity and flammability and also cause expense in their recovery, but the production on a continuous basis of coated supports, ready for irradiation, would be facilitated.
We have found that this object can be achieved by the use of certain substances which contain in the molecule two kinds of groups through which photopolymerisation can occur at rates which differ considerably from one another. The groups are chosen so that photopolymerisation of a layer of a liquid composition occurs rapidly to form a solid, essentially tack-free layer, which is, however, still soluble in certain solvents. When desired, parts of the layer are further subjected to a substantially greater amount of actinic radiation and photocrosslinking takes place through the other type of group in the already photopolymerised molecules of the layer, the parts of the layer which undergo photocrosslinking becoming much more resistant to solution in the solvents.
Unpublished experiments by the Applicants have shown that numerous potentially useful compounds, containing two types of units normally capable of undergoing photopolymerisation, do not give satisfactory results in such a process, photopolymerisation in the first stage being much retarded, apparently as a consequence of the presence in the molecule of another type of photopolymerisable unit, despite the incorporation of a variety of photoinitiators and photosensitisers. In other unpublished experiments of the Applicants, mixtures of two compounds, one containing a unit normally capable of undergoing photopolymerisation and the other having a different unit normally capable of undergoing photocrosslinking, gave unsatisfactory results, apparently due to inhibition of the photocrosslinking reaction.
It has been found that the desired process can be achieved by employing a compound which contains in the same molecule both at least one acryloyl or methacryloyl group and at least one anthryl group.
U.S. Pat. No. 2,980,535 describes the production of photopolymerisable polymers incorporating anthracene moieties. Thus, light-sensitive polymers are obtained by acetalisation of polyvinyl alcohol with anthracene-9-aldehyde: in order to increase the solubility of the product in, e.g., chlorinated aliphatic hydrocarbons, it is treated with acetic anhydride to give an acetylated acetal containing units of formula ##STR1##
In other Examples polyvinyl acetate is partially saponified and an acetal is formed with anthracene-9-aldehyde, or polyvinyl butyral is transacetalised with anthracene-9-aldehyde. There is no suggestion of forming a still photocrosslinkable product containing anthracene moieties by photopolymerising a compound containing both acryloyl or methacryloyl groups and anthracene moieties.
British Pat. No. 940,878 describes electrophotographic material comprising an electrically conductive support having a photoconductive coating which includes a polymer of an aromatic or heterocyclic ester of acrylic acid or of an alpha-alkyl acrylic acid. The polymer contains units of formula ##STR2## where R.sub.1 represents a substituted or unsubstituted aryl, aralkyl, heterocyclic or heterocyclic alkyl residue and R.sub.2 represents a hydrogen atom or an alkyl residue. Preferred residues R.sub.1 include those derived from anthracene; suitable monomers can be prepared by reacting a hydroxyl-containing anthracene derivative such as 2-hydroxyanthracene, 9-hydroxyanthracene or 9-hydroxymethylanthracene with acryloyl chloride. The polymers may be obtained by conventional polymerisation processes using catalysts such as peroxides or azo compounds. The photoconductive coating is formed by applying the polymer to the support in the form of a solution in an organic solvent or an aqueous or non-aqueous dispersion, and then removing the liquid medium by evaporation. In the formation of an electrophotographic image, the coating is charged by means of a corona discharge and then exposed behind a master to UV light emitted by high pressure mercury vapour lamps; it is then dusted with a developer which adheres to the parts not affected by light during the exposure to render visible an image corresponding to the master.
French Patent No. 2 272 107 discloses anthracenic monomers of formula ##STR3## where R.sub.3 and R.sub.4 each represent a hydrogen atom or an alkyl group and X and Y each represent a hydrogen, chlorine, or bromine atom or an alkyl or phenyl group, and the preparation of such monomers. The 2-anthryl monomers were polymerised by standard free radical-initiated addition polymerisation techniques. It is stated that it is desirable to protect the monomers from electromagnetic radiation during polymerisation, because it is well known that electronically excited anthracene groups can inhibit free radical polymerisation. The polymers produced are said to be high molecular weight materials suitable for use in photoconductive layers in electrophotographic processes. The preferred polymers are homopolymers of 2-anthrylmethyl methacrylate and 1-(2-anthryl)ethyl methacrylate and copolymers of these methacrylates with methyl methacrylate.
The inventor in the foregoing patent also described the preparation and polymerisation of the 2-anthryl substituted monomers in Macromolecules, 1975, 8 (1), 8-9. In referring to the problems associated with the polymerisation of vinyl monomers containing anthracene groups, he stated (i) that the extreme reactivity of the anthracene group towards free radicals made it impossible to synthesise a high molecular weight polymer of 9-vinylanthracene, (ii) that anthracene did not interfere with the radical polymerisation of methyl methacrylate in the absence of UV radiation, and (iii) that light-induced polymerisation of methyl methacrylate in the presence of benzoyl peroxide was also inhibited by anthracene. He proceeded from these statements to relate that he assumed that the methacrylate group attached to anthracene by a covalent bond should be polymerisable. He then described the experiments providing the correctness of that assumption; the polymerisation of 1-(2-anthryl)ethyl methacrylate was carried out whilst protecting the reactants from light.
G. I. Lashkov et al., in Doklady Akademii Nauk SSR, 1974, 214 (4), 850-853 described the photodimerisation of polymers containing anthracene groups in solution in dioxane. The polymers mentioned (their preparation is not described) include a homopolymer of 9-anthrylmethyl methacrylate and a copolymer thereof with methyl methacrylate.
U.S. Pat. No. 3,807,999 discloses the photo-cycloaddition polymerisation of bis-anthracenes of formula A--(CH.sub.2).sub.q OCOR.sub.5 COO(CH.sub.2).sub.q --A or A--COOR.sub.5 OCO--A, where each A denotes a 9-anthryl group, R.sub.5 denotes an alkylene group, and q is 1 or 2. Polymerisation is effected by exposing to ultraviolet radiation at 365 nm. The polymerisation is said to be reversible: the polymers formed can be degraded by heating or by irradiation at 254 nm and the degraded products can be repolymerised by irradiation at 365 nm. Most of the polymers are said to be soluble in haloalkanes; where they become insoluble in these solvents as a result of crystallisation, they are found to be soluble in dimethyl sulphoxide; this is said to prove that no crosslinked structures are formed. A method of recording information is described in which a polymer formed by photo-cycloaddition is applied to a support and imagewise exposed to heat, for instance using an infrared laser. The image thus formed can be scanned with a UV laser and recorded with a photocell sensitive to the eximer-fluorescence occurring in those places where the polymer has been degraded by the infrared laser.
U.S. Pat. No. 3,131,060 discloses photoconductive vinyl polymers having recurring groups of formula EQU --CH.sub.2 --C(R.sub.8)--R.sub.6 --(CH.dbd.CH).sub.r-1 --CO(CH.dbd.CH).sub.s --R.sub.7 --
where R.sub.6 represents an aromatic or heterocyclic nucleus, R.sub.7 represents an aromatic nucleus such as an anthracene nucleus, or a heterocyclic nucleus; R.sub.8 represents a hydrogen atom or a lower alkyl group: and r and s are each 1 or 2. The polymers can be prepared by various techniques, initiation occurring by free radicals, ion formation or radiation with actinic light. Polymerisation may also be carried out in situ on a support. An example describes the reaction of poly(vinylacetophenone) and 9-anthraldehyde to give a polymer of the above formula where R.sub.6 represents a benzene nucleus, R.sub.7 represents an anthracene nucleus, R.sub.8 represents a hydrogen atom, and r and s are 1.
U.S. Pat. No. 3,113,022 also discloses photoconductive polymers of the type disclosed in the foregoing patent, as well as many other types of photoconductive polymer. The use of the polymers in electrophotographic processes is described. It is stated that in such processes, insolubilisation of the polymers does not occur, or, if it does occur, does not play a role of any importance in the image-recording process; the illumination necessary for obtaining the conductive latent image is much smaller than that necessary for inducing crosslinking of the polymers.
In a recently published U.S. Pat. (No. 4,291,118) there is described a method for relief imaging a liquid polymeric composition, in which a layer of the composition is subjected to a first polymerisation treatment by exposure to actinic radiation, chemical hardening occurring sufficient to solidify the layer, then the solidified mass is further reacted with polymerising actinic radiation in a pattern, such that the solidified mass in the pattern is in a different chemical condition from that not in the pattern, and finally the portion solidified only by the first exposure to actinic radiation is selectively removed, such as by a solvent, to leave as a relief image the portion of the solidified mass which had been subjected to the further polymerising actinic radiation.
The only polymeric compositions named are conventional polyene-polythiol mixtures and in the Example such a mixture which is described as being commercially available is employed. There is no suggestion of using a compound containing two chemically different groups through which photopolymerisation and subsequently crosslinking could be achieved.